Subject: Re: [PATCH v2 2/4] powerpc/64s: Clear gprs on interrupt routine entry on Book3S

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 28 Nov 2022 11:52:59 +1000

On Mon Nov 7, 2022 at 1:32 PM AEST, Rohan McLure wrote:
> Zero user state in gprs (assign to zero) to reduce the influence of user
> registers on speculation within kernel syscall handlers. Clears occur
> at the very beginning of the sc and scv 0 interrupt handlers, with
> restores occurring following the execution of the syscall handler.
>
> Zero GPRS r0, r2-r11, r14-r31, on entry into the kernel for all
> other interrupt sources. The remaining gprs are overwritten by
> entry macros to interrupt handlers, irrespective of whether or not a
> given handler consumes these register values.
>
> Prior to this commit, r14-r31 are restored on a per-interrupt basis at
> exit, but now they are always restored on 64bit Book3S. Remove explicit
> REST_NVGPRS invocations on 64-bit Book3S. 32-bit systems do not clear
> user registers on interrupt, and continue to depend on the return value
> of interrupt_exit_user_prepare to determine whether or not to restore
> non-volatiles.
>
> The mmap_bench benchmark in selftests should rapidly invoke pagefaults.
> See ~0.8% performance regression with this mitigation, but this
> indicates the worst-case performance due to heavier-weight interrupt
> handlers. This mitigation is able to be enabled/disabled through
> CONFIG_INTERRUPT_SANITIZE_REGISTERS.

I think it looks good. You could put those macros into a .h file shared
by exceptions-64s.S and interrupt_64.S. Also interrupt_64.S could use
the HANDLER_RESTORE_NVGPRS macro to kill a few ifdefs I think? The
IMSR_R12 change *could* be done in a separate patch, if you're doing
another spin... sorry for the late feedback.

Reviewed-by: Nicholas Piggin <[email-masked]>

>
> Signed-off-by: Rohan McLure <[email-masked]>
> ---
> Resubmitting patches as their own series after v6 partially merged:
> Link: https://lore.kernel.org/all/[email-masked]/t/
>
> v2: REST_NVGPRS should be conditional on mitigation in scv handler. Fix
> improper multi-line preprocessor macro in interrupt_64.S
> ---
>  arch/powerpc/kernel/exceptions-64s.S | 47 +++++++++++++++++++++-----
>  arch/powerpc/kernel/interrupt_64.S   | 36 ++++++++++++++++++++
>  2 files changed, 74 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> index 651c36b056bd..0605018762d1 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -21,6 +21,19 @@
>  #include <asm/feature-fixups.h>
>  #include <asm/kup.h>
>  
> +/*
> + * macros for handling user register sanitisation
> + */
> +#ifdef CONFIG_INTERRUPT_SANITIZE_REGISTERS
> +#define SANITIZE_ZEROIZE_NVGPRS()	ZEROIZE_NVGPRS()
> +#define SANITIZE_RESTORE_NVGPRS()	REST_NVGPRS(r1)
> +#define HANDLER_RESTORE_NVGPRS()
> +#else
> +#define SANITIZE_ZEROIZE_NVGPRS()
> +#define SANITIZE_RESTORE_NVGPRS()
> +#define HANDLER_RESTORE_NVGPRS()	REST_NVGPRS(r1)
> +#endif /* CONFIG_INTERRUPT_SANITIZE_REGISTERS */
> +
>  /*
>   * Following are fixed section helper macros.
>   *
> @@ -111,6 +124,7 @@ name:
>  #define ISTACK		.L_ISTACK_\name\()	/* Set regular kernel stack */
>  #define __ISTACK(name)	.L_ISTACK_ ## name
>  #define IKUAP		.L_IKUAP_\name\()	/* Do KUAP lock */
> +#define IMSR_R12	.L_IMSR_R12_\name\()	/* Assumes MSR saved to r12 */
>  
>  #define INT_DEFINE_BEGIN(n)						\
>  .macro int_define_ ## n name
> @@ -176,6 +190,9 @@ do_define_int n
>  	.ifndef IKUAP
>  		IKUAP=1
>  	.endif
> +	.ifndef IMSR_R12
> +		IMSR_R12=0
> +	.endif
>  .endm
>  
>  /*
> @@ -502,6 +519,7 @@ DEFINE_FIXED_SYMBOL(\name\()_common_real, text)
>  	std	r10,0(r1)		/* make stack chain pointer	*/
>  	std	r0,GPR0(r1)		/* save r0 in stackframe	*/
>  	std	r10,GPR1(r1)		/* save r1 in stackframe	*/
> +	ZEROIZE_GPR(0)
>  
>  	/* Mark our [H]SRRs valid for return */
>  	li	r10,1
> @@ -544,8 +562,14 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	std	r9,GPR11(r1)
>  	std	r10,GPR12(r1)
>  	std	r11,GPR13(r1)
> +	.if !IMSR_R12
> +	ZEROIZE_GPRS(9, 12)
> +	.else
> +	ZEROIZE_GPRS(9, 11)
> +	.endif
>  
>  	SAVE_NVGPRS(r1)
> +	SANITIZE_ZEROIZE_NVGPRS()
>  
>  	.if IDAR
>  	.if IISIDE
> @@ -577,8 +601,8 @@ BEGIN_FTR_SECTION
>  END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
>  	ld	r10,IAREA+EX_CTR(r13)
>  	std	r10,_CTR(r1)
> -	std	r2,GPR2(r1)		/* save r2 in stackframe	*/
> -	SAVE_GPRS(3, 8, r1)		/* save r3 - r8 in stackframe   */
> +	SAVE_GPRS(2, 8, r1)		/* save r2 - r8 in stackframe   */
> +	ZEROIZE_GPRS(2, 8)
>  	mflr	r9			/* Get LR, later save to stack	*/
>  	LOAD_PACA_TOC()			/* get kernel TOC into r2	*/
>  	std	r9,_LINK(r1)
> @@ -696,6 +720,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
>  	mtlr	r9
>  	ld	r9,_CCR(r1)
>  	mtcr	r9
> +	SANITIZE_RESTORE_NVGPRS()
>  	REST_GPRS(2, 13, r1)
>  	REST_GPR(0, r1)
>  	/* restore original r1. */
> @@ -1441,7 +1466,7 @@ ALT_MMU_FTR_SECTION_END_IFCLR(MMU_FTR_TYPE_RADIX)
>  	 * do_break() may have changed the NV GPRS while handling a breakpoint.
>  	 * If so, we need to restore them with their updated values.
>  	 */
> -	REST_NVGPRS(r1)
> +	HANDLER_RESTORE_NVGPRS()
>  	b	interrupt_return_srr
>  
>  
> @@ -1667,7 +1692,7 @@ EXC_COMMON_BEGIN(alignment_common)
>  	GEN_COMMON alignment
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  	bl	alignment_exception
> -	REST_NVGPRS(r1) /* instruction emulation may change GPRs */
> +	HANDLER_RESTORE_NVGPRS() /* instruction emulation may change GPRs */
>  	b	interrupt_return_srr
>  
>  
> @@ -1733,7 +1758,7 @@ EXC_COMMON_BEGIN(program_check_common)
>  .Ldo_program_check:
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  	bl	program_check_exception
> -	REST_NVGPRS(r1) /* instruction emulation may change GPRs */
> +	HANDLER_RESTORE_NVGPRS() /* instruction emulation may change GPRs */
>  	b	interrupt_return_srr
>  
>  
> @@ -1751,6 +1776,7 @@ INT_DEFINE_BEGIN(fp_unavailable)
>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>  	IKVM_REAL=1
>  #endif
> +	IMSR_R12=1
>  INT_DEFINE_END(fp_unavailable)
>  
>  EXC_REAL_BEGIN(fp_unavailable, 0x800, 0x100)
> @@ -2164,7 +2190,7 @@ EXC_COMMON_BEGIN(emulation_assist_common)
>  	GEN_COMMON emulation_assist
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  	bl	emulation_assist_interrupt
> -	REST_NVGPRS(r1) /* instruction emulation may change GPRs */
> +	HANDLER_RESTORE_NVGPRS() /* instruction emulation may change GPRs */
>  	b	interrupt_return_hsrr
>  
>  
> @@ -2384,6 +2410,7 @@ INT_DEFINE_BEGIN(altivec_unavailable)
>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>  	IKVM_REAL=1
>  #endif
> +	IMSR_R12=1
>  INT_DEFINE_END(altivec_unavailable)
>  
>  EXC_REAL_BEGIN(altivec_unavailable, 0xf20, 0x20)
> @@ -2433,6 +2460,7 @@ INT_DEFINE_BEGIN(vsx_unavailable)
>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>  	IKVM_REAL=1
>  #endif
> +	IMSR_R12=1
>  INT_DEFINE_END(vsx_unavailable)
>  
>  EXC_REAL_BEGIN(vsx_unavailable, 0xf40, 0x20)
> @@ -2494,7 +2522,7 @@ EXC_COMMON_BEGIN(facility_unavailable_common)
>  	GEN_COMMON facility_unavailable
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  	bl	facility_unavailable_exception
> -	REST_NVGPRS(r1) /* instruction emulation may change GPRs */
> +	HANDLER_RESTORE_NVGPRS() /* instruction emulation may change GPRs */
>  	b	interrupt_return_srr
>  
>  
> @@ -2522,7 +2550,8 @@ EXC_COMMON_BEGIN(h_facility_unavailable_common)
>  	GEN_COMMON h_facility_unavailable
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  	bl	facility_unavailable_exception
> -	REST_NVGPRS(r1) /* XXX Shouldn't be necessary in practice */
> +	/* XXX Shouldn't be necessary in practice */
> +	HANDLER_RESTORE_NVGPRS()
>  	b	interrupt_return_hsrr
>  
>  
> @@ -2748,7 +2777,7 @@ EXC_COMMON_BEGIN(altivec_assist_common)
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  #ifdef CONFIG_ALTIVEC
>  	bl	altivec_assist_exception
> -	REST_NVGPRS(r1) /* instruction emulation may change GPRs */
> +	HANDLER_RESTORE_NVGPRS() /* instruction emulation may change GPRs */
>  #else
>  	bl	unknown_exception
>  #endif
> diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
> index a019ed6fc839..a262fe6964f3 100644
> --- a/arch/powerpc/kernel/interrupt_64.S
> +++ b/arch/powerpc/kernel/interrupt_64.S
> @@ -13,6 +13,20 @@
>  #include <asm/ppc_asm.h>
>  #include <asm/ptrace.h>
>  
> +/*
> + * macros for handling user register sanitisation
> + */
> +#ifdef CONFIG_INTERRUPT_SANITIZE_REGISTERS
> +#define SANITIZE_ZEROIZE_SYSCALL_GPRS()	\
> +	ZEROIZE_GPR(0);			\
> +	ZEROIZE_GPRS(5, 12);		\
> +	ZEROIZE_NVGPRS()
> +#define SANITIZE_RESTORE_NVGPRS()	REST_NVGPRS(r1)
> +#else
> +#define SANITIZE_ZEROIZE_SYSCALL_GPRS()
> +#define SANITIZE_RESTORE_NVGPRS()
> +#endif /* CONFIG_INTERRUPT_SANITIZE_REGISTERS */
> +
>  	.align 7
>  
>  .macro DEBUG_SRR_VALID srr
> @@ -96,6 +110,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	 * but this is the best we can do.
>  	 */
>  
> +	/*
> +	 * Zero user registers to prevent influencing speculative execution
> +	 * state of kernel code.
> +	 */
> +	SANITIZE_ZEROIZE_SYSCALL_GPRS()
>  	bl	system_call_exception
>  
>  .Lsyscall_vectored_\name\()_exit:
> @@ -124,6 +143,7 @@ BEGIN_FTR_SECTION
>  	HMT_MEDIUM_LOW
>  END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  
> +	SANITIZE_RESTORE_NVGPRS()
>  	cmpdi	r3,0
>  	bne	.Lsyscall_vectored_\name\()_restore_regs
>  
> @@ -159,7 +179,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	ld	r4,_LINK(r1)
>  	ld	r5,_XER(r1)
>  
> +#ifndef CONFIG_INTERRUPT_SANITIZE_REGISTERS
>  	REST_NVGPRS(r1)
> +#endif
>  	REST_GPR(0, r1)
>  	mtcr	r2
>  	mtctr	r3
> @@ -275,6 +297,11 @@ END_BTB_FLUSH_SECTION
>  	wrteei	1
>  #endif
>  
> +	/*
> +	 * Zero user registers to prevent influencing speculative execution
> +	 * state of kernel code.
> +	 */
> +	SANITIZE_ZEROIZE_SYSCALL_GPRS()
>  	bl	system_call_exception
>  
>  .Lsyscall_exit:
> @@ -315,6 +342,7 @@ BEGIN_FTR_SECTION
>  	stdcx.	r0,0,r1			/* to clear the reservation */
>  END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
>  
> +	SANITIZE_RESTORE_NVGPRS()
>  	cmpdi	r3,0
>  	bne	.Lsyscall_restore_regs
>  	/* Zero volatile regs that may contain sensitive kernel data */
> @@ -342,7 +370,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  .Lsyscall_restore_regs:
>  	ld	r3,_CTR(r1)
>  	ld	r4,_XER(r1)
> +#ifndef CONFIG_INTERRUPT_SANITIZE_REGISTERS
>  	REST_NVGPRS(r1)
> +#endif
>  	mtctr	r3
>  	mtspr	SPRN_XER,r4
>  	REST_GPR(0, r1)
> @@ -408,9 +438,11 @@ interrupt_return_\srr\()_user: /* make backtraces match the _kernel variant */
>  _ASM_NOKPROBE_SYMBOL(interrupt_return_\srr\()_user)
>  	addi	r3,r1,STACK_FRAME_OVERHEAD
>  	bl	interrupt_exit_user_prepare
> +#ifndef CONFIG_INTERRUPT_SANITIZE_REGISTERS
>  	cmpdi	r3,0
>  	bne-	.Lrestore_nvgprs_\srr
>  .Lrestore_nvgprs_\srr\()_cont:
> +#endif
>  	std	r1,PACA_EXIT_SAVE_R1(r13) /* save r1 for restart */
>  #ifdef CONFIG_PPC_BOOK3S
>  .Linterrupt_return_\srr\()_user_rst_start:
> @@ -424,6 +456,7 @@ _ASM_NOKPROBE_SYMBOL(interrupt_return_\srr\()_user)
>  	stb	r11,PACAIRQHAPPENED(r13) # clear out possible HARD_DIS
>  
>  .Lfast_user_interrupt_return_\srr\():
> +	SANITIZE_RESTORE_NVGPRS()
>  #ifdef CONFIG_PPC_BOOK3S
>  	.ifc \srr,srr
>  	lbz	r4,PACASRR_VALID(r13)
> @@ -493,9 +526,11 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
>  	b	.	/* prevent speculative execution */
>  .Linterrupt_return_\srr\()_user_rst_end:
>  
> +#ifndef CONFIG_INTERRUPT_SANITIZE_REGISTERS
>  .Lrestore_nvgprs_\srr\():
>  	REST_NVGPRS(r1)
>  	b	.Lrestore_nvgprs_\srr\()_cont
> +#endif
>  
>  #ifdef CONFIG_PPC_BOOK3S
>  interrupt_return_\srr\()_user_restart:
> @@ -585,6 +620,7 @@ _ASM_NOKPROBE_SYMBOL(interrupt_return_\srr\()_kernel)
>  	stb	r11,PACAIRQHAPPENED(r13) // clear the possible HARD_DIS
>  
>  .Lfast_kernel_interrupt_return_\srr\():
> +	SANITIZE_RESTORE_NVGPRS()
>  	cmpdi	cr1,r3,0
>  #ifdef CONFIG_PPC_BOOK3S
>  	.ifc \srr,srr
> -- 
> 2.34.1